Lithium doping/undoping in disordered coke carbons

The lithium doping/undoping behaviour of negative electrodes composed of heat-treated pitch coke with a disordered structure is investigated by using 7Li NMR and X-ray diffraction. A hysteresis is observed in the change in the average layer spacing d(002) between the carbon sheets during the charge/discharge process. This change increases proportionally with the charge to a level of about 300 mAh/g but then stays nearly constant throughout the remainder of the charging to 773 mAh/g capacity. It decreases linearly, in proportion to the lithium undoping during the subsequent discharging. The 7Li NMR spectra suggest that two types of lithium doping sites exist in the coke: one type (S-site) has a wide range of spinning side bands while the other (C-site) has a narrower range. It is considered that C-site lithium atoms are located between the carbon sheets, while S-site lithium atoms are located at the surface of the microcrystallites formed by these sheets or at their microcavities. The C-site lithium atoms are far easier to dope/undope than the S-site counterparts.